The mechanisms of recirculation of thymus-dependent lymphocytes will be studied in rats transfused with radiolabeled thoracic duct lymphocytes. The role of surface receptors in lymphocyte homing into lymph nodes will be examined. Experiments will be performed to determine if lymphocytes have receptors specific for recognition of specialized postcapillary venules of lymph nodes or for hormones or pharmacologic agents in the microenvironment of these venules. Myxoparamyxoviruses which react with different sites on lymphocytes will be used to study the relationship of lymphocyte determinants to the distribution of the cells in the body. In vitro systems to study the interaction between lymphocytes and specialized post-capillary venules and the traffic of lymphocytes through the parenchyma of lymph nodes will be explored.